Most angiosperm species are induced to flower in response to environmental stimuli such as day length and temperature, and internal cues, such as age. Adult organs of flowering plants develop from groups of stem cells called meristems. The identity of a meristem is inferred from structures it produces: vegetative meristems give rise to roots and leaves, inflorescence meristems give rise to flower meristems, and flower meristems give rise to floral organs such as sepals and petals. Not only are meristems capable of generating new meristems of different identity, but their own identity can change during development. For example, a vegetative shoot meristem can be transformed into an inflorescence meristem upon floral induction, and in some species, the inflorescence meristem itself will eventually become a flower meristem. Despite the importance of meristem transitions in plant development, little is known about the underlying mechanisms.
Following germination, the shoot meristem produces a series of leaf meristems on its flanks. However, once floral induction has occurred, the shoot meristem switches to the production of flower meristems. Flower meristems produce floral organ primordia, which develop individually into sepals, petals, stamens or carpels. Thus, flower formation can be thought of as a series of distinct developmental steps, i.e. floral induction, the formation of flower primordia and the production of flower organs. Mutations disrupting each of the steps have been isolated in a variety of species, suggesting that a genetic hierarchy directs the flowering process (see for review, Weigel and Meyerowitz, In Molecular Basis of Morphogenesis (ed. M. Bernfield). 51st Annual Symposium of the Society for Developmental Biology, pp. 93-107, New York, 1993).
Recently, studies of two distantly related dicotyledons, Arabidopsis thaliana and Antirrhinum majus, led to the identification of three classes of homeotic genes, acting alone or in combination to determine floral organ identity (Bowman, et al., Development, 112:1, 1991; Carpenter and Coen, Genes Devl., 4:1483, 1990; Schwarz-Sommer, et al., Science, 250:931, 1990). Several of these genes are transcription factors whose conserved DNA-binding domain has been designated the MADS box (Schwarz-Sommer, et al., supra).
Earlier acting genes that control the identity of flower meristems have also been characterized. Flower meristems are derived from inflorescence meristems in both Arabidopsis and Antirrhinum. Two factors that control the development of meristematic cells into flowers are known. In Arabidopsis, the factors are the products of the LEAFY gene (Weigel, et al., Cell 69:843, 1992) and the APETALA1 gene (Mandel, et al., Nature 360:273, 1992). When either of these genes is inactivated by mutation, structures combining the properties of flowers and inflorescence develop (Weigel, et al., supra; Irish and Sussex, Plant Cell, 2:741, 1990). In Antirrhinum, the homologue of the Arabidopsis LEAFY gene is FLORICAULA (Coen, et al., Cell, 63:1311, 1990) and that of the APETALA1 gene is SQUAMOSA (Huijser, et al., EMBO J., 11:1239, 1992). The latter pair contains MADS box domains.
Flowering plants exhibit one of two types of inflorescence architecture: indeterminate, in which the inflorescence grows indefinitely, or determinate, in which a terminal flower is produced. In two mutants in distantly related species, terminal flower 1 in Arabidopsis and centroradialis in Antirrhinum, inflorescences that are normally indeterminate are converted to a determinate architecture. The Antirrhinum gene CENTRORADIALIS (CEN) and the Arabidopsis gene TERMINAL FLOWER 1 (TFL1) were shown to be homologous, which suggests that a common mechanism underlies indeterminacy in these plants. However, unlike CEN, TFL1 is also expressed during the vegetative phase, where it delays the commitment to inflorescence development and thus affects the timing of the formation of the inflorescence meristem as well as its identity.
There is increasing incentive by those working in the field of plant biotechnology to successfully genetically engineer plants, including the major crop varieties. One genetic modification that would be economically desirable would be to accelerate the flowering time of a plant. Induction of flowering is often the limiting factor for growing crop plants. One of the most important factors controlling induction of flowering is day length, which varies seasonally as well as geographically. There is a need to develop a method for controlling and inducing flowering in plants, regardless of the locale or the environmental conditions, thereby allowing production of crops, at any given time. Since most crop products (e.g. seeds, grains, fruits), are derived from flowers, such a method for controlling flowering would be economically invaluable.